1. Technical Field
This invention relates generally to photoconductor electrophotography. I have invented a single-layer, positive-charging, organic photoconductor material with low pigment loading for liquid toner electrophotography.
2. Background Art
In electrophotography, a latent image is created on the surface of an insulating, photoconducting material by selectively exposing areas of the surface to light. A difference in electrostatic charge density is created between the areas on the surface exposed and unexposed to light. The latent electrostatic image is developed into a visible image by electrostatic toners containing pigment components and thermoplastic components. The toners are selectively attracted to the photoconductor surface either exposed or unexposed to light, depending on the relative electrostatic charges of the photoconductor surface, development electrode and the toner. The photoconductor may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles. For laser printers, the preferred embodiment is that the photo-conductor and toner have the same polarity, but different levels of charge.
A sheet of paper or intermediate transfer medium is given an electrostatic charge opposite that of the toner and passed close to the photoconductor surface, pulling the toner from the photoconductor surface onto the paper or intermediate medium still in the pattern of the image developed from the photoconductor surface. A set of fuser rollers melts and fixes the toner in the paper, subsequent to direct transfer, or indirect transfer when using an intermediate transfer medium, producing the printed image.
There is a demand in the laser printer industry for multi-colored images. Responding to this demand, designers have turned to liquid toners, with pigment components and thermoplastic components dispersed in a liquid carrier medium, usually special hydrocarbon liquids. With liquid toners, it has been discovered, the basic printing colors--yellow, magenta, cyan and black, may be applied sequentially to a photoconductor surface, and from there to a sheet of paper or intermediate medium to produce a multi-colored image.
The important photoconductor surface, therefore, has been the subject of much research and development in the electrophotography art. A large number of photoconductor materials have been disclosed as being suitable for the electrophotographic photoconductor surface. For example, inorganic compounds such as amorphous silicon (Si), arsenic selenide (AS.sub.2 Se.sub.3), cadmium sulfide (CdS), selenium (Se), titanium oxide (TiO.sub.2) and zinc oxide (ZnO) function as photoconductors. However, these inorganic materials do not satisfy modern requirements in the electro-photography art of low production costs, high-speed response to laser diode or other light-emitting-diode (LED) and safety from non-toxicity.
Therefore, recent progress in the electrophotography art with the photoconductor surface has been made with organic materials as organic photoconductors (OPC). Typically, the OPC's in the current market are of the negative-charging type with a thin charge generation material layer beneath a thicker charge transport material layer deposited on top of the charge generation layer. The negative-charging OPC's perform well for xerographic copiers and printers in the following applications:
a. Low end (4-10 copies per minute) and high end (more than 50 copies per minute) xerographic systems using dry powder developers of one or two colors, or using liquid developers for black and white copies only; and, PA1 b. High image quality (above 1800 DPI) color proofing, lithographic plate printing and master xero-printing systems with life expectancies of less than 100 cycles. PA1 1. Large amounts of ozone are generated in the negative corona-charging process, creating environmental concerns. This problem has been addressed by installing ozone absorbers like activated carbon filters, and by using contact negative-charging instead of corona- charging. These ozone remediation approaches, however, have drawbacks of their own and are not attractive commercial solutions. PA1 2. Negative corona-charging generally results in less charge pattern uniformity compared to positive corona-charging. Lower charge pattern uniformity in turn results in more noise and less definition in the final image. PA1 3. In liquid toner processes, designers have been able to develop more charge stability in positively charged toners than in negatively charged toners. Therefore, positive-charging OPC's are preferred for a discharged area developed image as in laser printers. PA1 M=hydrogen (metal free), Cu, Mg, Zn, TiO, VO, InY (Y=halogen, Cl, Br, I, F) PA1 X=halogen (Cl, Br, I, F), nitro --(NO.sub.2), cyano (--CN), sulfonyl --(RSO.sub.2 NH.sub.2), alkyl, alkoxy, and PA1 n=0-4. PA1 Ar=phenyl, naphthyl, biphenyl or ter-phenyl groups, and PA1 R=alkyl and alkoxy groups, and EQU --(O--C-phenyl-N-phenyl).sub.x (II) PA1 x=1-10
However, prior art negative-charging OPC's also have several drawbacks, namely:
From the prior art it is known that most of the phthalocyanines (Pc) may serve as photoconductors. Also, it is known to disperse phthalocyanines as a charge generation material in a polymeric binder matrix which serves as a charge transport material. However, these approaches, for single-layer photoconductors with low Pc loadings, for example 1-30 wt. %, have been used only in low end (less than 10 copies per minute) and high end (more than 50 copies per minute) dry powder developers, and not in liquid toner environments.
Specific morphologies of phthalocyanine pigment powder have been known to exhibit excellent photoconductivity. These phthalocyanine pigments have been used as a mixture in polymeric binder matrices in electrophotographic photoconductors, deposited on a conductive substrate. In these phthalocyanine/binder photoconductors, the photogeneration of charge and the charge transport occur in the particles of the phthalocyanine pigment while the binder is inert. Therefore, the photoconductor may be made of a single layer of phthalocyanine/binder. These single-layer photoconductors are known to be very good positive-charging OPC's due to the hole (positive charge) transportability of the phthalocyanine pigment.
In these single-layer photoconductors, then, there is no need to add charge transport molecules, nor to have a separate charge transport layer. The phthalocyanine pigment content may be in the range of about 10-30 wt. %, high enough to perform both charge generation and charge transport functions, with the binder content being in the range of about 90-70 wt. %. The single photoconductor layer is usually more than about 3 microns (um) thick in order to achieve the required charge acceptance and resulting image contrast. In any event, it is more than 1 micron thick which is the maximum thickness for charge generation layers in multi-layer photoconductors.
Also, it is known to use phthalocyanine pigment as a charge generation component in a multi-layer photoconductor. Today, the commercially available OPC for digital electrophotography, wherein the writing head is LED array or laser diode, uses such a multi-layer photoconductor. The charge generation layer containing the phthalocyanine pigment is less than 1 micron (um) thick. A charge transport layer about 20-30 microns (um) thick and containing transport molecules other than the phthalocyanine pigment, is overcoated on top of the charge generation layer.
These types of multi-layer OPC's, however, are only used as negative-charging ones, so they have all the drawbacks of negative-charging OPC's discussed above. So, there remains a strong incentive for the development of a phthalocyanine pigment type positive-charging OPC.
It is known to use a positive-charging OPC for liquid toner electrophotography. This generic OPC, however, is very slow due to its low surface energy density (50-1000 ergs/cm.sup.2), and has a very short life (less than 100 cycles) before its charge acceptance and photo-discharge capabilities deteriorate. This OPC then, is limited to slow, short-term applications like color proofing.
Also, it is known to use a positive-charging OPC made from copper phthalocyanine pigments (Pc) of a specific crystal form imbedded in a cross-linkable binder. These photoconductors have high Pc loadings, for example, in the range of about 10-30 wt. %. Also, the pigments are metal chelate phthalocyanines which are considered hazardous materials, reducing the industrial attractiveness of this OPC. Also, the specific phthalocyanine crystal form is unstable, and, after a change in the crystal form, the OPC has a low response to laser diode light sources in the 780-830 nm range, further reducing the attractiveness of this OPC for laser printer applications.
It is also known to use an improved positive-charging OPC with a thin film (less than 500 Angstroms) of diamond-like tetrahedrally bonded materials like amorphous silicon (Si), silicon carbide (SiC) and silicon nitrile (SIN) added by plasma deposition. This manufacturing method, however, is very expensive, so this OPC is not economically suitable for low end (less than 10 copies per minute) applications.
Therefore, there is a need in the liquid toner electrophotography art for a novel, single-layer positive-charging OPC with low Pc loadings, for example 1-30 wt. %, exhibiting high speed and long life. Preferably, the high speed capability is obtained by a photoconductor of low activation energy of less than 10 ergs/cm.sup.2 required for discharging it in the active wave length region of infrared (IR) laser LED (600 nm-900 nm). In the prior art, this high speed capability has been obtained by certain infrared absorber pigments or dyes, including phthalocyanine compounds, dispersed in a charge transport medium as discussed above. If these pigments are of the specific crystal form which exhibits both charge generation and charge transport capability, then the OPC may be made from them simply by dispersing the IR absorbing phthalocyanine pigment in a binder matrix.
However, for these types of positive-charging OPC's, there is no data which supports their performance stability in liquid toner systems. My expectation is that the effect of liquid toners, especially those preferred by the industry which contain charge control agents, will be to contaminate the surface of the phthalocyanine pigment and binder only OPC's, resulting in positive surface charge deterioration of the OPC's, and limits on their feasibility in the high speed, high volume applications in the range above 10 copies per minute.
So, there is a need in the liquid toner electro-photography art for a novel, single-layer positive-charging OPC containing low loadings, for example, 1-30 wt. %, phthalocyanine pigment and exhibiting chemical and electrical stability. One response by the industry to this incentive has been to investigate a positive-charging, multi-layer OPC with an electron transport molecule in the upper layer which must be an electron acceptor molecule and an electron transporter molecule under the application of a positive electric field. See, for example, the disclosure of U.S. Pat. No. 4,559,287 (McAneney, et al.). These types of OPC's use derivatives of fluorenylidene methane, for example, as the electron acceptor and transport molecule. These types of molecules, however, exhibit poor solubility, resulting in recrystallization in the OPC forming mixture during coating, poor compatibility with popular binders, and poor reaction yield resulting in high production costs. Also, these types of molecules tend to be highly carcinogenic, resulting in safety risks to workers and users and therefore, low market receptivity.
Also, U.S. Pat. No. 5,087,540 (Murakami et al.) discloses a positive-charging, single-layer photo-conductor for electrophotography which has X-type and/or T-type phthalocyanine compound dispersed partly in a molecular state and partly in a particulate state in a binder resin. To make the dispersion, the phthalocyanine compound is agitated in a solvent with the binder resin for from several hours to several days. This approach, therefore, has manufacturing drawbacks.
Another response by the industry to the incentive for the development of a phthalocyanine type positive-charging OPC has been to investigate a multi-layer OPC wherein the relative positions of the charge generation and transport layers are reversed. See, for example, the disclosure of U.S. Pat. No. 4,891,288 (Fujimaki et al.). These types of OPC's, however, require a protective overcoat to avoid mechanical damage to the OPC because the upper pigment-containing layer is very vulnerable to the development component, the transfer medium component and the cleaning component in the electrophotographic system. These overcoat layers have problems of their own, increasing the residual voltage of the photoconductor and increasing its electrical instability. See, for example, the disclosures of U.S. Pat. Nos. 4,923,775 (Schank) and 5,069,993 (Robinette, et al.).
Therefore, it is an object of this invention to provide a low loading, single-layer phthalocyanine type positive-charging OPC which exhibits stable electrical properties, including charge acceptance, dark decay and photodischarge, in a high cycle, high severity, liquid toner electrophotographic process. Modern digital imaging systems wherein the writing head is LED array or laser diode, have very high light intensities (about 100 ergs/cm.sup.2) over very short exposure time spans (less than 50 nano seconds), resulting in severe conditions for the OPC compared to optical input copiers with light intensities between about 10-30 ergs/cm.sup.2 and exposure times between about several hundred micro-seconds to mili-seconds.
Unfortunately, there is no product on the market today which provides such stable electrical properties. This is because the phthalocyanine type positive-charging OPC exhibits instability when it is frequently exposed to the corona charger and the intense light source in the liquid toner electrophotographic process. I have discovered this instability to be more pronounced at the strong absorption, high light intensity, short exposure time conditions required for the liquid toner laser printing process. The instability is exhibited in the significant increase of the dark decay after a small number of repeat cycles of laser printing. Also, the instability is exhibited in the decrease in surface potential. These instabilities cause deleterious changes in image contrast, and raise the issue of the reliability of image quality.
Also, I have discovered that these instabilities in the phthalocyanine/binder photoconductor seem to be independent of the chemical structure or morphology of the pigment. Instead, they appear to be dependent on the nature of the contact between individual pigment particles. These observations of mine have been made only recently, and there is no report or suggestion in the prior art about how to effectively address and solve the problem of photoconductor instability.
Preferably, desirable electrophotographic performance may be defined as high charge acceptance of about 30-100 V/um.sup.2, low dark decay of less than about 5 V/sec., and photodischarge of at least 70% of surface charge with the laser diode beam of 78 mn or 83 mn frequency, through the optical system including beam scanner and focus lenses, synchronized at 0.05 micro seconds for each beam.
I have discovered that this type of OPC may be obtained by a combination of special phthalocyanine pigments and sensitizers embedded in a polymeric binder. The sensitizers are chemically stable transport molecules which do not induce charge injection from the OPC surface into its center when it is frequently exposed to liquid toner, and they are compatible with the polymer binder.